HTTP and API Basics in Python with requests

Most modern applications need to talk to other systems over the internet, and HTTP is the protocol almost all of that communication happens over. Python's third-party requests library is the most widely used tool for making HTTP calls because it wraps the lower-level built-in urllib module in a much simpler, more readable interface — installed with pip install requests.

Understanding a handful of core ideas — HTTP methods, status codes, and JSON — is enough to consume the vast majority of web APIs you'll encounter.

Why it matters

Programs get weather data, take payments, send messages and talk to AI models by calling web APIs. The steps are always the same: build a URL with parameters, send a request, check the status code and parse the JSON that comes back. This lesson is the foundation for both using APIs and, later, building them with FastAPI.

GET and POST Requests

A GET request retrieves data from a server and should not change anything on the server side — it's what happens whenever you visit a webpage or fetch data from an API endpoint. A POST request sends data to a server, typically to create or modify something, such as submitting a form or creating a new record. With requests, these look like requests.get(url, params={...}) for query parameters and requests.post(url, json={...}) for sending a JSON request body.

Status Codes

Every HTTP response includes a numeric status code summarizing what happened. Codes in the 200s mean success (200 OK is the most common). Codes in the 300s indicate redirection. Codes in the 400s mean the client made a bad request (404 Not Found, 401 Unauthorized, 400 Bad Request). Codes in the 500s mean the server itself failed while processing an otherwise valid request. Checking response.status_code, or calling response.raise_for_status() (which raises an exception for 4xx/5xx responses), is essential before trusting a response's contents.

Parsing JSON Responses

Most modern web APIs return data formatted as JSON (JavaScript Object Notation), a lightweight text format that maps naturally onto Python dictionaries and lists. Calling response.json() on a requests response object parses the JSON body and returns it as native Python data structures, ready to use directly — no manual string parsing required. If the fallback urllib module is used instead of requests, you would need to manually decode the response bytes and pass them through Python's built-in json.loads() function to get the same result.

Examples

import requests

response = requests.get(
    "https://api.example.com/users",
    params={"active": "true", "limit": 2}
)

response.raise_for_status()  # raises an exception for 4xx/5xx responses
data = response.json()

print(response.status_code)
print(type(data))
print(data)

200
<class 'list'>
[{'id': 1, 'name': 'Asha', 'active': True}, {'id': 2, 'name': 'Ravi', 'active': True}]

import requests

# --- Using requests (typical, third-party) ---
payload = {"title": "New Task", "done": False}
response = requests.post("https://api.example.com/tasks", json=payload)
print(response.status_code)
print(response.json())

# --- Using urllib (standard library fallback, no installation needed) ---
import json
import urllib.request

data_bytes = json.dumps(payload).encode("utf-8")
req = urllib.request.Request(
    "https://api.example.com/tasks",
    data=data_bytes,
    headers={"Content-Type": "application/json"},
    method="POST",
)
with urllib.request.urlopen(req) as resp:
    result = json.loads(resp.read().decode("utf-8"))
    print(result)

201
{'id': 42, 'title': 'New Task', 'done': False}
{'id': 42, 'title': 'New Task', 'done': False}

Exercise

Without using the network, practise the three steps around a request. Build the full URL for a GET request with two query parameters. Parse the JSON body of a response and print the name inside it. Write describe, which classifies a status code as success, client error or server error.

Expected output: https://api.example.com/users?page=2&limit=10, Asha, client error (one per line)

import json
from urllib.parse import urlencode

base = "https://api.example.com/users"
params = {"page": 2, "limit": 10}
# TODO: print the full URL with the query string

body = '{"data": {"name": "Asha"}}'
# TODO: parse the body and print the name


def describe(status):
    # TODO: 200-299 -> "success", 400-499 -> "client error", 500-599 -> "server error"
    pass


print(describe(404))

import json
from urllib.parse import urlencode

base = "https://api.example.com/users"
params = {"page": 2, "limit": 10}
print(f"{base}?{urlencode(params)}")

body = '{"data": {"name": "Asha"}}'
print(json.loads(body)["data"]["name"])


def describe(status):
    if 200 <= status < 300:
        return "success"
    if 400 <= status < 500:
        return "client error"
    return "server error"


print(describe(404))

Quick quiz

Interview questions